Automatic fitting for a visual prosthesis

ABSTRACT

A method of automatically adjusting an electrode array to the neural characteristics of an individual patient is disclosed. By recording neural response to a predetermined input stimulus, one can alter that input stimulus to the needs of an individual patient. A minimum input stimulus is applied to a patient, followed by recording neural response in the vicinity of the input stimulus. By alternating stimulation and recording at gradually increasing levels, one can determine the minimum input that creates a neural response, thereby identifying the threshold stimulation level. One can further determine a maximum level by increasing stimulus until a predetermined maximum neural response is obtained.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This patent application is a divisional application of U.S. patentapplication Ser. No. 10/864,590, filed Jun. 8, 2004, the disclosure ofwhich is incorporated herein by reference.

GOVERNMENT RIGHTS NOTICE

This invention was made with government support under grant No.R24EY12893-01, awarded by the National Institutes of Health. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention is generally directed to neural stimulation andmore specifically to an improved method of adjusting neural stimulationlevels for artificial vision.

BACKGROUND OF THE INVENTION

In 1755 LeRoy passed the discharge of a Leyden jar through the orbit ofa man who was blind from cataract and the patient saw “flames passingrapidly downwards.” Ever since, there has been a fascination withelectrically elicited visual perception. The general concept ofelectrical stimulation of retinal cells to produce these flashes oflight or phosphenes has been known for quite some time. Based on thesegeneral principles, some early attempts at devising a prosthesis foraiding the visually impaired have included attaching electrodes to thehead or eyelids of patients. While some of these early attempts met withsome limited success, these early prosthetic devices were large, bulkyand could not produce adequate simulated vision to truly aid thevisually impaired.

In the early 1930's, Foerster investigated the effect of electricallystimulating the exposed occipital pole of one cerebral hemisphere. Hefound that, when a point at the extreme occipital pole was stimulated,the patient perceived a small spot of light directly in front andmotionless (a phosphene). Subsequently, Brindley and Lewin (1968)thoroughly studied electrical stimulation of the human occipital(visual) cortex. By varying the stimulation parameters, theseinvestigators described in detail the location of the phosphenesproduced relative to the specific region of the occipital cortexstimulated. These experiments demonstrated: (1) the consistent shape andposition of phosphenes; (2) that increased stimulation pulse durationmade phosphenes brighter; and (3) that there was no detectableinteraction between neighboring electrodes which were as close as 2.4 mmapart.

As intraocular surgical techniques have advanced, it has become possibleto apply stimulation on small groups and even on individual retinalcells to generate focused phosphenes through devices implanted withinthe eye itself. This has sparked renewed interest in developing methodsand apparatuses to aid the visually impaired. Specifically, great efforthas been expended in the area of intraocular retinal prosthesis devicesin an effort to restore vision in cases where blindness is caused byphotoreceptor degenerative retinal diseases such as retinitis pigmentosaand age related macular degeneration which affect millions of peopleworldwide.

Neural tissue can be artificially stimulated and activated by prostheticdevices that pass pulses of electrical current through electrodes onsuch a device. The passage of current causes changes in electricalpotentials across visual neuronal membranes, which can initiate visualneuron action potentials, which are the means of information transfer inthe nervous system.

Based on this mechanism, it is possible to input information into thenervous system by coding the information as a sequence of electricalpulses which are relayed to the nervous system via the prostheticdevice. In this way, it is possible to provide artificial sensationsincluding vision.

One typical application of neural tissue stimulation is in therehabilitation of the blind. Some forms of blindness involve selectiveloss of the light sensitive transducers of the retina. Other retinalneurons remain viable, however, and may be activated in the mannerdescribed above by placement of a prosthetic electrode device on theinner (toward the vitreous) retinal surface (epiretial). This placementmust be mechanically stable, minimize the distance between the deviceelectrodes and the visual neurons, and avoid undue compression of thevisual neurons.

In 1986, Bullara (U.S. Pat. No. 4,573,481) patented an electrodeassembly for surgical implantation on a nerve. The matrix was siliconewith embedded iridium electrodes. The assembly fit around a nerve tostimulate it.

Dawson and Radtke stimulated cat's retina by direct electricalstimulation of the retinal ganglion cell layer. These experimentersplaced nine and then fourteen electrodes upon the inner retinal layer(i.e., primarily the ganglion cell layer) of two cats. Their experimentssuggested that electrical stimulation of the retina with 30 to 100 uAcurrent resulted in visual cortical responses. These experiments werecarried out with needle-shaped electrodes that penetrated the surface ofthe retina (see also U.S. Pat. No. 4,628,933 to Michelson).

The Michelson '933 apparatus includes an array of photosensitive deviceson its surface that are connected to a plurality of electrodespositioned on the opposite surface of the device to stimulate theretina. These electrodes are disposed to form an array similar to a “bedof nails” having conductors which impinge directly on the retina tostimulate the retinal cells. U.S. Pat. No. 4,837,049 to Byers describesspike electrodes for neural stimulation. Each spike electrode piercesneural tissue for better electrical contact. U.S. Pat. No. 5,215,088 toNorman describes an array of spike electrodes for cortical stimulation.Each spike pierces cortical tissue for better electrical contact.

The art of implanting an intraocular prosthetic device to electricallystimulate the retina was advanced with the introduction of retinal tacksin retinal surgery. De Juan, et al. at Duke University Eye Centerinserted retinal tacks into retinas in an effort to reattach retinasthat had detached from the underlying choroid, which is the source ofblood supply for the outer retina and thus the photoreceptors. See,e.g., E. de Juan, et al., 99 Am. J. Ophthalmol. 272 (1985). Theseretinal tacks have proved to be biocompatible and remain embedded in theretina, and choroid/sclera, effectively pinning the retina against thechoroid and the posterior aspects of the globe. Retinal tacks are oneway to attach a retinal array to the retina. U.S. Pat. No. 5,109,844 tode Juan describes a flat electrode array placed against the retina forvisual stimulation. U.S. Pat. No. 5,935,155 to Humayun describes aretinal prosthesis for use with the flat retinal array described in deJuan.

In addition to the electrode arrays described above, there are severalmethods of mapping a high resolution camera image to a lower resolutionelectrode array. U.S. Pat. No. 6,400,989 to Eckmiller describesspatio-temporal filters for controlling patterns of stimulation in anarray of electrodes. The assignee of the present applications has threerelated U.S. patent applications: Ser. No. 09/515,373, filed Feb. 29,2000, entitled Retinal Color Prosthesis for Color Sight Restoration;Ser. No. 09/851,268, filed May 7, 2001, entitled Method, Apparatus andSystem for Improved Electronic Acuity and Perceived Resolution Using EyeJitter Like Motion; and Attorney Docket S242-USA, filed on current dateherewith, entitled User Directed Pixel Re-Mapping. All threeapplications are incorporated herein by reference.

Each person's response to neural stimulation differs. In the case ofretinal stimulation, a person's response varies from one region of theretina to another. In general, the retina is more sensitive closer tothe fovea. Any stimulation, less than the threshold of perception, isineffective. Stimulation beyond a maximum level will be painful andpossibly dangerous to the patient. It is therefore, important to map anyvideo image to a range between the minimum and maximum for eachindividual electrode. With a simple retinal prosthesis, it is possibleto adjust the stimulation manually by stimulating and questioning thepatient. As resolution increases, it is tedious or impossible to adjusteach electrode by stimulating and eliciting a patient response.

A manual method of fitting or adjusting the stimulation levels of anauditory prosthesis is described in U.S. Pat. No. 4,577,642, Hochmair etal. Hochmair adjusts the auditory prosthesis by having a user compare areceived signal with a visual representation of that signal.

A more automated system of adjusting an auditory prosthesis using middleear reflex and evoked potentials is described in U.S. Pat. No.6,157,861, Faltys et al. An alternate method of adjusting an auditoryprosthesis using the stapedius muscle is described in U.S. Pat. No.6,205,360, Carter et al. A third alternative using myogenic evokedresponse is disclosed in U.S. Pat. No. 6,415,185, Maltan.

U.S. Pat. No. 6,208,894, Schulman describes a network of neuralstimulators and recorders implanted throughout the body communicatingwirelessly with a central control unit. U.S. Pat. No. 6,522,928,Whitehurst, describes an improvement on the system described in Schulmanusing function electro stimulation also know as adaptive deltamodulation to communicate between the implanted devices and the centralcontrol unit.

The greatest dynamic range is achieved by setting the minimumstimulation at the threshold of perception and the maximum stimulationlevel approaching the pain threshold. It is unpleasant for a patient tofirst concentrate to detect the minimum perception and then be subjectedto stimulation near the threshold of pain.

The human retina includes about four million individual photoreceptors.An effective visual prosthesis may include thousands of electrodes. Anautomated system is needed to adjust individual electrodes in a visualprosthesis for maximum benefit without the need for patient interactionin a long and difficult process.

SUMMARY OF THE INVENTION

The invention is a method of automatically adjusting a retinal electrodearray to the neural characteristics of an individual patient. Byrecording neural response to a predetermined input stimulus, one canalter that input stimulus to the needs of an individual patient. Aminimum input stimulus is applied to a patient, followed by recordingneural response in the vicinity of the input stimulus. By alternatingstimulation and recording at gradually increasing levels, one candetermine the minimum input that creates a neural response, therebyidentifying the threshold stimulation level. One can further determine amaximum level by increasing stimulus until a predetermined maximumneural response is obtained.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will be best understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the preferred retinal prosthesis for implementing thepresent invention.

FIG. 2 is a flow chart showing the process of auto fitting an electrodearray.

FIG. 3 depicts a block diagram of the retinal prosthesis electroniccontrol unit.

FIG. 4 is a graph depicting a typical neural response to electricalinput.

FIG. 5 depicts an alternate retinal prosthesis using cortical recording.

FIG. 6 depicts an alternate retinal prosthesis using iris recording.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims.

FIG. 1 shows the preferred retinal prosthesis. A stimulating electrodearray 10 is placed against the outer surface of a retina 12(epiretinally). A cable 14 pierces a sclera 16 and attaches to anelectronic control unit 18. The electronic control unit is attached tothe sclera and moves with the sclera. A return electrode 20 may beplaced outside the sclera and distant from the retina 12. Alternatively,electrodes in the electrode array 10 may be used a return electrodes.Electricity travels through the body between the stimulating electrodearray 10 and return electrode 20, to complete an electrical circuit. Thestimulating electrode array 10 is a plurality of tiny electrodes. Eachelectrode on the stimulating electrode array 10 is as small as possibleto maximize the effect of electrical current on the retina, and to fitthe maximum number of electrodes on the retina. The return electrode 20,if used, may be quite large by comparison. A coil 22 surrounds thesclera just inside the conjunctiva and acts as an antenna to send andreceive data from an external unit (not shown). A matching coil ismounted in a pair of glasses along with a camera for collecting a videoimage. Power to operate the control unit may also be provided throughthe coil 22.

The electronics described herein may be in the electronics control unit18 or mounted externally and communicate through the coil 22. Anexternal solution may initially be simpler and less expensive. Withimprovements in integrated circuits, it will be cost effective toinclude all of the control functions described herein within the controlunit 18. An entirely implanted solution would greatly reduce the timerequired to complete the fitting process.

FIG. 2 is a flow chart of the automatic fitting sequence. In the flowchart, the value N is the current (or selected) electrode, X is theneural activity recorded, and L is the level of stimulation. First N isset to 0 40 and them incremented 42. The first electrode, electrode N,is addressed 44. The stimulation level is set to zero 46, and thenincremented 48. The neural tissue is stimulated at the minimum level 50.The stimulation is immediately followed by a recording of activity inthe neural tissue 52. Alternatively, recording can be donesimultaneously by an adjacent electrode. If recording is donesimultaneously, one must be careful to distinguish between neuralactivity and electrical charge from the stimulating electrode. Theneural response follows stimulation (see FIG. 4). Simultaneousstimulation and recording requires that the recording phase be longerthan the stimulation phase. If so, the stimulation and neural responsecan be separated digitally. If the recorded neural activity is less thana predetermined level 54, the stimulation level is increased and steps48-54 are repeated.

In most cases, the preset minimum level is any measurable neuralactivity. However, perception by the patient is the determining factor.If neural activity is detected and the patient reports no perception,the minimum level must be set higher. Once minimum neural activity isrecorded, the stimulation level is saved in memory 56. The level is thenfurther increased 58 and stimulation is repeated 60. Again stimulationis immediately followed by recording neural activity 62. If apredetermined maximum level has not been reached, steps 58-64 arerepeated until the predetermined maximum stimulation level is obtained.Once the predetermined maximum stimulation level is obtained, steps42-64 are repeated for the next electrode. The process is continueduntil a minimum and maximum stimulation level is determined for eachelectrode 66.

The maximum stimulation level borders on discomfort for the patient.Because the automatic fitting process is automated, high levels ofstimulation are only applied for a few microseconds. This significantlydecreases the level of discomfort for the patient compared withstimulating long enough to elicit a response from the patient.

The fitting process is described above as an incremental process. Thefitting process may be expedited by more efficient patterns. For examplechanges may be made in large steps if it the detected response issignificantly below the desired response, followed by increasingly smallsteps as the desired response draws near. The system can jump above andbelow the desired response dividing the change by half with each step.

Often, neural response in a retina is based, in part, geographically.That is, neurons closer to the fovea require less stimulation thanneurons farther from the fovea. Hence once a stimulation is level is setfor an electrode, one can presume that the level will be similar for anadjacent electrode. The fitting process may be expedited by starting ata level near the level set for a previously fit adjacent electrode.

Automating the fitting process has many advantages. It greatly expeditesthe process reducing the efforts of the patient and clinician. Further,the automated process is objective. Patient responses are subjective andmay change over time due to fatigue. In some cases, a patent may not beable to provide the required responses due to age, disposition, and/orlimited metal ability.

FIG. 3 depicts a block diagram of the control unit. The block diagram isa functional diagram. Many of the functional units would be implementedin a microprocessor. A control unit 80 sets and increments a counter 82to control the stimulation level of the stimulator 84. The stimulationsignal is multiplexed in MUX 86 to address individual electrodes 88.After each stimulation, the addressed electrode returns a neuralactivity signal to a recorder 90. The signal is compared to the storedminimum or maximum level (stored in a memory 92) in a comparator 94.After programming, a signal from a video source 96, or other neuralstimulation source, is adjusted in a mapping unit 98, in accordance withthe minimum and maximum levels stored in the memory 92. The adjustedsignal is sent to the stimulator 84, which in synchronization with MUX86 applies the signal to the electrodes 88. The electronics for thecontrol unit could be external or within the implanted prosthesis.

FIG. 4 is a graphical representation of the neural response toelectrical stimulus. This figure is derived from actual recordings of afrog retina. Response in a human retina will be similar. The verticalaxis is current while the horizontal axis is time. Four curves 100-106show the response at varying input current levels. An input pulse 108,is followed by a brief delay 110, and a neural response 112. Hence, itis important to properly time the detecting function. Either thestimulating electrode must be switched to a detecting electrode duringthe brief delay or detecting must occur on another electrode andcontinue long enough to record the neural response. It should also benoted that the delay period 110 becomes shorter with increasedstimulation current. Hence, the system must switch faster fromstimulation mode to detecting mode with increased current. The change indelay time may also be used as an additional indication of neuralresponse. That is, the minimum and maximum may be determined by matchingpredetermined delay times rather than predetermined output levels. Asstimulation increases, it becomes more useful to employ an alternaterecording means as described in the following alternate embodiments.

In a first alternate embodiment, the recording electrode may be corticalelectrode mounted on or near the visual cortex. Temporary externalelectrodes placed on the scalp proximate to the visual cortex may recordneural activity in the visual cortex. This allows the system to accountfor any variations in neural processing between the retina and thevisual cortex. It, however, requires electrodes either implanted in thevisual cortex or placed temporarily near the visual cortex. Thisalternate embodiment may be combined with the preferred embodiment byfirst using cortical electrodes to perform an initial fitting of theprosthesis in a clinic. Thereafter, retinal recording may be used toreadjust the prosthesis for any changes over time.

FIG. 5 shows the first alternate retinal prosthesis. A stimulatingelectrode array 150 is placed against the outer surface of a retina 152(epiretinally). A cable 154 pierces a sclera 156 and attaches to anelectronic control unit 158. A return electrode 160 may be placeddistant from the retina 152. The stimulating electrode array 150 is aplurality of tiny electrodes. One or more recording electrodes 162 areplaced in near the visual cortex. The recording electrodes may temporaryexternal electrodes, implanted electrodes under the scalp, or electrodeimplanted within the visual cortex.

In a second alternate embodiment, the recording electrode may be eitherimplanted in the iris, or placed externally near the iris. The irisresponds to light, or the perception of light. In response to anincrease in electrical stimulation the iris will contract because thebody perceives an increase in light entering the eye. Conversely, theiris expands in response to a decrease in electrical stimulation. Whilethe response of the iris is relatively slow, the neurological signalsinitiating a change in the iris respond quickly. Measuring these signalsmay provide alternate feed back as to the body's response to theelectrical stimulus. Alternatively, an optical device aimed at the eyemay detect the movement of the iris.

FIG. 6 shows the second alternate retinal prosthesis. A stimulatingelectrode array 210 is placed against the outer surface of a retina 212(epiretinally). A cable 214 pierces a sclera 216 and attaches to anelectronic control unit 218. A return electrode 220 may be placeddistant from the retina 212. The stimulating electrode array 210 is aplurality of tiny electrodes. A recording electrode 224 is place in theperiphery of the iris sensing electrical stimulus to the iris.

In a third alternate device, electroluminescent pigments may be appliedto the retina. Electroluminescent pigments cause an individual cell toglow when it fires it neuro-electrical charge. A camera of the type usedfor retinal photos may detect neural response by detecting theelectroluminescent glow of the applied pigment.

Accordingly, what has been shown is an improved method of stimulatingneural tissue for increased resolution. While the invention has beendescribed by means of specific embodiments and applications thereof, itis understood that numerous modifications and variations could be madethereto by those skilled in the art without departing from the spiritand scope of the invention. It is therefore to be understood that withinthe scope of the claims, the invention may be practiced otherwise thanas specifically described herein.

1. A visual stimulator comprising: a variable output stimulator coupledto an electrode suitable for contact with retinal tissue, the variableoutput stimulator adapted to send a stimulation signal to the retinaltissue; a neural activity recorder coupled to an electrode suitable forcontact with neural tissue, the neural activity recorder recording aneural activity signal in response to the stimulation signal; a memorystoring a minimum neural activity level and a maximum neural activitylevel; and a control device for controlling said stimulator in responseto a comparison between the neural activity signal recorded by saidrecorder and the minimum and maximum neural activity levels stored insaid memory.
 2. The visual stimulator according to claim 1, wherein saidneural activity recorder is a visual cortex activity recorder.
 3. Thevisual stimulator according to claim 1, wherein said neural activityrecorder is an iris sphincter activity recorder.
 4. The visualstimulator according to claim 1, wherein said memory also storesadditional neural activity levels as a consequence of the comparison. 5.The visual stimulator according to claim 1, wherein said minimum neuralactivity level is the lowest stimulation level where neural activity isdetected.
 6. The visual stimulator according to claim 1, wherein saidelectrode coupled to said stimulator is adjacent to said electrodecoupled to said recorder.
 7. The visual stimulator according to claim 1,wherein said electrode is suitable for implantation in an eye.
 8. Thevisual stimulator according to claim 4, wherein said additional neuralactivity levels are a first neural activity level equal to or just abovethe minimum neural activity level and a second neural activity levelequal to or just above the maximum neural activity level.
 9. The visualstimulator according to claim 8, wherein said stimulator is adapted tobe in a fitting mode for storing the first neural activity level and thesecond neural activity level in the memory, and in an operative mode forstimulating visual neurons between a minimum level corresponding to thefirst neural activity level and a maximum level corresponding to thesecond neural activity level.
 10. The visual stimulator according toclaim 9, wherein said operative mode occurs according to an externalinput.
 11. The visual stimulator according to claim 10, wherein saidexternal input is a video capture device.
 12. A visual prosthesiscomprising: a variable output stimulator coupled to a plurality ofelectrodes suitable for contact with visual neurons; a neural activityrecorder coupled to a plurality of electrodes suitable for contact withvisual neurons; a first memory section for storing a preset minimumlevel of neural activity and a preset maximum level of neural activity;a control device for controlling said stimulator according to saidrecorder, and determining a minimum level of stimulation causing saidminimum level of neural activity and a maximum level of stimulationcausing said maximum level of neural activity; and a second memorysection for storing said minimum level of stimulation and said maximumlevel of stimulation.
 13. The visual prosthesis according to claim 12,wherein said minimum level of stimulation and said maximum level ofstimulation are determined separately for each electrode.
 14. The visualprosthesis according to claim 12, further comprising a video capturedevice coupled to said control device for controlling said stimulatorbetween said minimum stimulation level and said maximum stimulationlevel according to a video image.
 15. The visual prosthesis according toclaim 12, wherein said minimum stimulation level and said maximumstimulation level are recalibrated on a periodic basis.